A 47-year-old patient complains of pain in the lumbar region. The patient was provisionally diagnosed with radiculitis and a course of physiotherapy was carried out. However, the patient's condition did not improve. X-ray of the spine and pelvis revealed osteoporosis and significant bone defects. Complete blood count revealed normochromic anemia and total protein of 107 g/L. General urinalysis revealed proteinuria of 4.0 g/L. What study is necessary to establish the final diagnosis in this case?